This invention relates to an electron beam irradiation device for carrying out welding or heat treatment. In particular it relates to an electron beam irradiation device constituted by partitioning, by an electron beam permeable window, the interior and exterior of a chamber for electron beam generation, which is maintained in a vacuum condition, and a method of manufacturing an electron beam permeable window.
Electron beam irradiation devices are employed, for example, in fixed recovery systems for NOx or SOx by induced chemical reaction of waste gases, or to effect the bridging of high molecular compounds.
In conventional electron beam irradiation devices, irradiation of a workpiece that is to be subjected to heat treatment is performed by inserting it into a chamber. Due to the need to prolong filament life etc, this chamber had to be maintained at a degree of vacuum of about 10.sup.-4 to 10.sup.-5 torr. The size of the workpiece is restricted since the chamber therefore could not be made very large.
However, by using an electron beam permeable window to partition the interior and exterior of the chamber, it has become possible to lead the electron beam out to the exterior through the electron beam permeable window so that electron beam irradiation could be effected in the atmosphere or a specified gas.
Titanium foil was conventionally employed as the material of this window for leading out the electron beam. Titanium is employed on account of its excellent electron permeability, high melting point, and the fact that it can be manufactured in thin foil a few tens of microns in thickness.
In order to confirm these characteristics of titanium, the inventors investigated the electron permeability, melting point, thermal conductivity and electrical resistance etc. of titanium in comparison with various other materials. The results of this investigation are shown in Table 1. In Table 1, M/(.rho..multidot.Z.sup.8/9) is a coefficient found from the maximum depth which the electron beam penetrates into the interior of a workpiece when the workpiece is irradiated by the electron beam, and expresses the transmittance of the electron beam. It is desirable that this transmittance of the material of the window should be as high as possible. And from the point of view of heat resistance, the melting point should also be as high as possible.
__________________________________________________________________________ M M P Z .rho. .multidot. Z.sup.8/9 (.degree.C.) (cal/9.degree. C.) (cal/cm-S-t) (.times.10.sup.-6 .OMEGA.-m) __________________________________________________________________________ K 19 0.86 39.1 0.849 760 0.177 0.74 6.15 Ca 20 1.55 40.1 0.485 838 0.149 0.3 3.91 Mg 12 1.74 24.3 0.405 650 0.245 0.367 4.45 P 15 1.83 30.97 0.388 44.25 0.177 -- 1 .times. 10.sup.7 Si 14 2.33 28.08 0.310 1410 0.162 0.20 10 Be 4 1.848 9.012 0.307 1277 0.45 0.35 4 C 6 2.25 12.01 0.293 3727 0.165 0.057 1375 Al 13 2.7 26.98 0.257 660 0.215 0.53 2.65 Ti 22 4.507 47.9 0.157 1668 0.124 0.053 42 Ge 32 5.32 72.6 0.133 937 0.073 0.14 46 V 23 6.1 50.9 0.115 1900 0.119 0.074 25 Zr 40 6.49 91.22 0.112 1852 0.067 0.211 40 Fe 26 7.87 55.8 0.0926 1536 0.11 0.18 9.71 Cu 27 8.96 63.55 0.0808 1083 0.092 0.941 1.67 __________________________________________________________________________
Furthermore, to lower heat emission, preferably the thermal conductivity should be high and the electrical resistance low. However, it may not be possible for a material to have both high electrical resistance and yet low thermal conductivity.
There are several materials that have better electron permeability than titanium. However, of these, potassium, calcium, magnesium, phosphorus and aluminium all have low melting points and so cannot be expected to be capable of standing up to the heat generated by the passage of the electron beam. Beryllium is toxic, carbon has very poor resistance to oxidation, and silicon is difficult to produce in the form of a thin film and is mechanically brittle. Because of this, the presently used titanium foil, while not necessarily representing the perfect solution, may be considered as being a comparatively satisfactory material.
However, notwithstanding that titanium foil has excellent electron permeability etc., due to its tendency to creep when heated by the thermions generated during passage of the electron beam, it undergoes severe corrosion damage by reaction with the atmosphere or special gas atmospheres outside the chamber. Also, titanium foil tends to be deformed or damaged by the difference in the internal and external pressure of the chamber. These reasons make prolonged use of an electron beam irradiation device at high output difficult.
In an electron beam irradiation device, cooling of the window is therefore carried out by providing the window with a cooling mechanism. However, there is the problem that the energy loss in performing cooling is considerable. That is, when titanium foil is heated by electron beam irradiation over a long period, parts which are in contact with the atmosphere or corrosive gases of a gas atmosphere are damaged and reduced in thickness. When such thinned titanium foil is heated to high temperature, creep is produced by the difference in internal and external pressures at the window. This may result in breakage of the titanium foil due to creep damage, with the risk of the atmosphere or gases entering the chamber, damaging the electron beam generating device.